1. Field of the Invention
This invention relates to a copying lens system, and more particularly to a wide angle copying lens system having a completely symmetrical arrangement about a diaphragm.
2. Description of the Prior Art
With the recent trend of miniaturization, higher speed, higher quality of picture and multifunction of copying machines, new requirements have been also added to the copying lens system.
To attain the miniaturization, it is necessary to make an optical system small. In order to make the optical system small, a lens system having a shorter focal length is required but when the focal length is made to be shorter with respect to the originals of the same size, a half field angle increases and the outer diameter of the lenses tends to increase. Conversely, when the overall length of the lens system is shortened, the outer diameter of the lenses can be decreased. That is, a lens system of a wide field angle and a short overall length becomes required.
For high speed copying, a bright lens system is required but when the lens system is made to be bright, the outer diameter of the lenses increases, which is contrary to the miniaturization of the lens system. However, recent development of a photosensitive material having high sensitivity has made it possible to use a lens system having a relatively large F number. In addition, such a lens system is often used with the usage of a high output lamp such as a xenon lamp.
The higher quality of picture is affected by the image forming performance of the lens system, developing performance of the machine and the like. Recently, there are problems in that the number of uses of copying from copied originals increases and various kinds of originals such as prints, hand-written originals and photographs, etc. are used.
As for the multifunction, it is a recent trend to employ a zoom system which can reproduce any size of copies. The lens system for such copying machines has to provide for the same level of image forming performance of unit-magnification at the time of enlargement and reduction.
In the past, the copying lens system having a completely symmetrical arrangement with respect o a diaphragm is often used. As one example, Japanese Patent Publication No. 10,091/76 discloses a lens system comprising six units in the form of six lenses, which has a good image forming performance. This lens system is bright but the image surface cannot be flatened as far as the vicinity of 30.degree. of half field angle, thus making it impossible to provide a wide field angle. In addition, the performance of this lens system is considerably deteriorated at the time of variation of the magnification, and cannot adapt to the variation of magnification from .times.0.5 to .times.2.0 including unit-magnification.
As a wide field angle lens system, there is one which is of a topogon type and has 34.degree. of half field angle, as disclosed in Japanese Patent Publication No. 29,410/68. In the lens system of this type, a radial curvature of field is easily corrected up to a wide angle but a difference of tangential image surface caused by color occurs as the field angle becomes wide and therefore, correction of aberrations in a wide range of wavelength becomes difficult and the lens system cannot be used in the whole visible range of wavelength. Furthermore, to attain the wide field angle, an air space between the second lens and the third lens has to be increased, thus posing a disadvantage in that the overall length of the lens is extended.